TESTING WITH ULTRA VIOLET (UV) LIGHT

How UV works, blacklight, wavelengths explained, several UV torches reviewed

 

BUY NOW: CLICK ON A PICTURE

 

Also, UV jewellers loupe

 

CONTENTS

Plain-English summary of using UV light to check antiques etc

Technical explanation

UV safety

Products guide

Examples of what you see under UV light

 

 

USING UV (ULTRA VIOLET) LIGHT TO CHECK ANTIQUES etc

Ultra violet light (known is the U.S.A. as 'black light') is a very intense blue light, it is not visible to the human eye. Fluorescent-tube UV torches give off a very dull purple or violet glow. LED UV torches give off a dull lilac-blue. But the actual UV light is invisible, so the only way you can tell how effective it is - is to observe its effects.

If porcelain has been repaired, the missing part will have been 'rebuilt' or the crack glued, and then carefully repainted so that the colour matches. The colour will match perfectly in white light (e.g. daylight) but under UV light the new paint shows up a totally different shade. This applies to many decorative items, watch dials, dolls heads, faint signatures on letters, modern touch-up paint on old paintings. You have probably noticed a similar effect when looking at a car parked under a yellow street light, new paint on the car can show up quite a different shade, whereas in daylight it matches perfectly.

Other examples in the world of antiques and art: oil paintings, art on paper, marble, jade and ivory, all show characteristic patterns under UV light:

 

APPEARANCE

UNDER UV LIGHT

 

 

INDICATES

 

Oil Paintings

Bluish-white

Dark bluish-violet

Chartreuse glaze

Very small blue dots

Dark blotches

Bright yellow

 


Lining compound

Picture putty

Old varnish

Dust

Repairs, new signatures & overpainting

Recent touch-up with white lead or Naples yellow

 

 

Art on Paper

Bright areas

Smudged areas

Very faint writing

 

 


New patches of paper

Erasers

Erased signature

 

 

Marble

Strong purple

Mottled white

 

 

 

Fresh cut

Old Marble

 

 

Jade

Intense colour

Mottled

 

 

 

 

Fresh carved

Old Jade

 

Ivory

Purple

Yellow tone

 

 


Newly carved

Old Ivory

 

Amber

 

Yes amber does fluoresce bluish under UV light, but so too will many substances so this test is not diagnostic.

The one reliable test is destructive, which is never recommended in gemmology. Touch the tip of a red-hot needle to it. If the burning smells acrid (like plastic) then it’s plastic; if the burning smells sweet, it’s amber.

Failing the burn-test, the best test is to drop it into heavily-salted water, amber will float, GENERALLY plastics sink. But there are many thousands of different types of plastic and I doubt that anyone has compared them all with amber, there might be some that float in salt water.

The above tests will not tell you if the amber has been treated to change its colour, neither will it distinguish ‘natural’ amber from ‘pressed’ / ‘reconstituted’ amber, since both are amber.

 

Uranium glass
(Vasceline Glass)

 

Glows spectacularly.

 

How does it work? It works by reflecting off the surface and showing up the difference in the chemical composition of the paint, or by showing characteristics of the item's basic chemical make-up.

Examples from every day life.

Many everyday substances glow under UV light: chlorophyll, teeth and antifreeze; olive oil, honey and ketchup; some cosmetics, some drugs, some postage stamps and some flowers.

Any object painted with UV paint will glow under UV light, common uses are finding gold balls, tortoises, even your keys...providing you will be searching for them in the dark.

There is starch in ordinary paper that makes it glow a vivid blue when lit with ultra violet light, but paper banknotes (which are made of a linen-type material, not paper) merely look 'slightly blue', hence UV lights are used as 'forged banknote detectors'; some paper banknotes (and all plastic banknotes) are also printed with 'secret' marks, using UV-ink, which can only be seen under UV light - see some examples at the bottom of this page.

Additives in soap powders glow under ultra violet light, they are added for that very reason: UV light in daylight makes your white washing glow blue-white, so that the advertisers can say that their powder 'washes whiter than white'.

Quinine is an ingredient of some drinks (e.g. tonic water), it gives them a distinctive bitter taste. Quinine glows under UV light. So the next time you are in a dimly-lit bar drinking a gin and tonic, shine your UV torch on it and see it glow.

Specialist Uses

Cross linkers and light boxes for viewing gels in biological laboratories; germicidal UV lights for treating food before it goes on sale in the supermarkets; welding operations; for curing uv-sensitive adhesives or uv-sensitive nail varnish; for inserting into pipes carrying water to fish tanks or fish ponds to kill algae and bacteria; for treating skin wounds (though there is a balance, here, between the benefit of stimulating healing and the risk of causing cancer).

Animals and plants

Humans need UV light (from sunlight) to produce vitamin D in the skin (nearly everyone in Northern Europe is vitamin-D deficient in the winter due to lack of sunshine!); in the summer it's popular to take advantage of prolonged UV light from the sun to burn the skin (it's called a suntan) - and if there's no sun there are tanning parlours in which you can be gently grilled under giant UV lights. Over-exposure to UV light can cause cancer but some exposure is essential, the solution is to go out in the sun (without sun cream) whenever possible but don't stay out long enough to get a tan.

In humans, the lens of the eye filters out UV light so that we cannot see it, but some people who have had eye operations (e.g. following a cataract operation) can see UV, it appears as a bluish or purplish glow. An example of this is in the paintings of water lilies by Monet following his cataract operation.

For most humans, the only way to see the effect of UV light is to use a UV torch. For instance, to detect lichen, shade it from daylight, shine a UV light on it, and see if it fluoresces white, bluish or orange - if so, it's probably lichen (try it out on Cladonia portentosa).

The same applies to freshly-spilt blood and urine, the human eye can't see it fluoresce in daylight, but you can see it glow under a UV light in the dark (used by the the forensic services at crime scenes).

Most mammals, including man, cannot see UV light, but many other animals can. Kestrels can detect the urine trails of prey; to reindeer living in the Arctic urine and lichens appear black against the ice and snow as they absorb UV light (urine is bad, it indicates nearby predators whilst lichens are good, reindeers eat them); recent research indicates that many fish, reptiles and birds can see UV light.

Here are four questions we get asked about UV light.

Question: so why do some UV lights appear really bright and others appear really dim?

Answer: many popular UV torches are 'popular' because they appear 'really bright' to the human eye, but the amount of actual UV light (which is not visible to the human eye) is very small, and its effect is very slight...to the point of being useless. By contrast, some UV torches appear dim to the human eye, but the amount of actual UV light is large and they work well. We have tested each of our products and each one in our store includes the test results.

 

Question: do I have to use a UV torch in the dark?

Answer: the effect is certainly spectacular if you go into a completely dark room, but usually it is sufficient to use it in dim light. Use a square of dark cloth, or the underneath of a table in a dim corner or even the inside of your jacket (you may have seen antiques dealers lifting their jacket and, it would seem, examining their armpit with a blue light). UV lights used in shops for testing banknotes include a shade. This is no different from using an ordinary torch, turn it on outside in bright sunlight and you won't see any effect.

Our two best models (UV-1 and UV-100) give out a huge amount of UV light and can be used in daylight, though not in direct sunlight. All UV lights work best in dim light, the dimmer the better.

 

Question: can't I just use a brighter UV torch, so that I don't have to use it in 'dim' light?

Answer: To an extent - yes. But it depends on the type (wavelength) of the light, if it is wrong (as with all the cheap UV torches) then it won't work, no matter how bright the UV light. Providing the type (wavelength) is right, then yes, a larger brighter UV torch will light up a wider area and work in brighter light, just the same as an ordinary torch. Scroll down or click here for recommendations.

Question: if I use it by shining the light on the surface of an antique, does that mean that it won't see below the surface?

Answer: Exactly so! If the item has been repaired and then re-glazed, a UV torch will be of no use, there will be no 'new' paint to see, the entire surface will be new. Similarly, you cannot see through layers of paint to see 'hidden paintings', UV light is not the same as x-rays. But you can see differences in surface paint, and that can indicate that a painting has been 'touched up'. Similarly, if a signature reacts differently to the surrounding work, it's clearly made of a different paint and may have been added.

 

TECHNICAL EXPLANATION

The electromagnetic spectrum ranges from AM at one end (which includes Medium Wave and Long Wave radio) to Gamma Rays at the other. The measurements on the right of the chart, below, are the length of each wave section (from peak to trough) in nanometers (nm). 1nm = 0.000001mm.

As you see from the chart, "visible light" falls between microwaves and x-rays.

UV light, although close to x-rays, cannot penetrate the human body, its effect (for shortwave UV) is limited to burning the surface of skin (and eyes). Having your skin burnt by shortwave UV light is very popular, it's called 'getting a suntan', and if there is no sun, you can get yourself grilled under giant UV lights on a sun bed.

 

The tiny band we call 'light' ranges from infrared (the human eye can't detect far-infrared) to ultraviolet (the human eye can't detect ultraviolet):

And within this, the tiny bit we call 'ultraviolet light' looks like this:

365nm - 302mn - 254nm

 

UV light is divided into three types depending on the wavelength (these classifications are approximate, since there's nothing to distinguish the boundary from one to another):

"Longwave" UV, also known as UV-A, 400 to 300nm

"Midrange" UV, also known as UV-B, 315 to 280nm

"Shortwave" UV, also known as UV-C, 280 to 250nm

All UV light is invisible to the human eye, the UV torches you buy also give out some visible light and it is this you can see as a dull glow. Manufacturers of the cheaper torches ensure that the 'dull' glow isn't too dull, so that the user says, "Wow, this UV torch is so powerful!". Giving out some visible light is, of course, useful, otherwise you wouldn't know if your UV torch was switched on or off.

All the UV torches we sell are UV-A / longwave, they are not shortwave (shortwave burns skin and can damage eyes, they are usually encased in safe 'viewing cabinets'). Here is a list of how the various types (wavelengths) of UV-A / long wave lights compare (my standard test is on British twenty pound note look for the "20" and also the myriad of fine lines by the portrait of the Queen)

UVA

410nm: technically this is UV, but it has no effect on a twenty pound note, it does not show up any of the security markings

395nm: the short lines are clearly visible but the "20" is not - not even if you use the it in the dark

385nm: the short lines are visible; the "20" is just about visible, but only in the dark.

365-370nm: both the short lines and the "20" are clearly visible.

Scroll down slightly, to see the wavelengths of all the models we sell.

 

SAFETY

UVA, SUMMARY. All our UV torches are UVA (see above). It is quite safe to point these at the skin, there is no way this type of UV can burn skin. However, please do not look directly into the light.

UVA, DETAIL. UVA is not harmful to the skin but could (possibly) be harmful to the eyes if exposed for prolonged periods. A "prolonged period' is measured as 'exposure over an eight hour period'. What we don't know is, how long is a 'prolonged period'? For more than one hour in any eight hour period? For more than 10mns in any eight hour period? For more than 1mn? We really don't know, therefore: - to avoid any possibility of harming your eyes, don't look directly into the light.

UVB and UVC (but mostly UVC) can burn the eyes or skin. When used in sun beds they are specifically designed to burn (tan) the skin, but the user must wear dark glasses to protect the eyes. Prolonged exposure, such as over-sunbathing, can lead to skin cancer. We do not sell UVB or UVC torches.

There are many other uses of UVC which do not concern the antiques and jewellery trades. Examples are: cross linkers and light boxes for viewing gels in biological laboratories; germicidal UV lights for treating food before it goes on sale in the supermarkets; controlling algae and bacteria in water; welding operations; for curing adhesives and nail varnishes.

Susceptibility to skin being burnt by UV light depends on skin colour. Brown-skinned people are less sensitive to being burnt by UV light, melanin in the skin provides protection; completely black-skinned people have a variation, eumelanin, which gives even greater protection. This dates from the time when most humans lived in Africa. In populations that spread northwards, to lands where there wasn't much sunshine, fair skin evolved; it evolved to be more sensitive to UV light, because UV light is necessary for producing vitamin D in the skin.

 

THE PRODUCTS

Scroll down to see examples of what you see under each of these lights - which gives you an excellent idea of how they compare.

 

420nm (estimated)

ref.UV-9
£9.50


spectacularly bright to the human eye but it doesn't emit much UV light. It does work. But it doesn't work very well; you can see the "5" on a five pound note, but it's faint - though it's better if you work in darkness.

 

 

 

410nm (estimated)

UV-KR2
£3.90


The worst of all worlds, a wavelength that is too long to work well, and the tiny light (one LED) will light up only a few millimeters area at a time. painful to use - but look on the positive side, it's only £3.90



 

365-370nm

UV-KR
£7.50

This works really well but covers only a tiny area (a few millimeters). Good to carry in your pocket, it's tiny, and includes a keyring.


 

 

365-370nm

ref.UV-12
£23.00


** Sold out, but we do (as of Summer 2017) have a handful of display models for sale at the Kempton fair **
A small neat torch which gives out a lot of UV light (12 LED lights)
this is by far the best for 'general purpose' for the price, it covers a good area (a few inches).



 

 

 

365nm

UV-white
£37.50

One end is a UV torch with 1 super-powerful UV L.E.D. The other end is an extraordinarily powerful ordinary (white light) torch, which is useful for inspecting antiques, e.g. shining white light through fine porcelain or into narrow spaces. The UV light is not only of a good wavelength (365nm) but is also finely-focused, the result is a UV light that is so powerful it even works in daylight (though the effect is more spectacular in dim light). It's powered by a built-in rechargeable battery, one charge gives up to 6 hours of use. If you need a white-light torch, then this is a good choice. However, if you only need a UV torch, then the next one (below) is neater and brighter.



 

 

365nm

ref. UV-1
£39.50


The wavelength is the same as the 'UV-white' above but it slightly brighter.
It's powered by one rechargeable 18650 battery (supplied)
and it comes with a power supply (you plug this directly into the torch,
you don't need a separate charger.
One charge lasts up to 8 hours of use. So if you want the most effective UV
and don't need an ordinary torch, this is most definitely the one to go for.


 

 

 

 

 

 

 

390-395nm

ref. UV-WHITE, £29.50

The effect of the 100 L.E.D. lights is to cover a huge area. An example of its coverage is in the picture below, in white light the shoes and laces are almost the same colour, but the makers have (not intentionally!) used shoelaces that glow. Imagine examining this same area with a keyring UV light, you would have to very slowly scan each millimeter.
If you want to impress your friends, this is the one to make them go, 'wow'.

Particularly good for special effects, or if you want to light up a showcase full of fluorescent items.

Its effectiveness (the 390-395nm wavelength) can be seen on a £20. banknote, the "20" is only just visible (i.e. this UV light isn't as good as our best, UV-1 and UV-WHITE, above) - but the other security marking on the note is vivid (see the selection of large photos when you view ref. UV-WHITE).

Go for this one if you want an exceptionally bright wide-area UV light and want to keep the budget under £30.00.

 

 

365-370nm

ref.UV
£9.50


On a twenty pound note both the short lines and the "20" are clearly visible.
It's not the smallest UV light (it's just about pocket-size) and the technology is old-fashioned (a fluorescent tube, not LED lights),
but it has the advantage that it really doesn't cost much, is of a reasonable brightness, and actually works quite well.
So if you're working to a tight budget and want something that is quite 'adequate'.
then go for this on. The same tube is used in the banknote detectors (below) used in shops.



 

 

365-370nm

uv-note-checker
£15.00


All the other models on this page are battery-powered, this one is mains-only,
for testing banknotes, includes a shade

 

 

 

EXAMPLES

Click on each picture to see it greatly enlarged

A TWENTY POUND NOTE

Here are examples of a British twenty pound note when viewed under various UV lights.

Click on each picture see it greatly enlarged.

 

UV9, £4.90

The light is so bright it's causing reflections, but none of the security markings are visible.
This is with the light held three inches away. At 12 inches the note appears quite dark and you can't really see anything at all.
True, the note shows up in a pretty blue colour, but it's not reacting to the UV light.

 

UV-KR, £8.50

The actual UV is OK, it does show up the "20", not very brightly, but it works.
The disadvantage is that you have to hold the light half an inch away and it only lights up a tiny area at a time, which is awful hard work.
The advantage is that it only costs £8.50 which is pretty good for a UV that actually works,
and it is fixed to keyring fitting, so you can keep it handy with your keys.

 

 

UV-12, £23.00

The light really isn't very bright but you can see the security markings beautifully.
This is with the light held three inches away, at 12 inches the entire note appears dark except for the "20" which still glows.
Please note, this item is discontinued, there are only a handful remaining.

 

 

UV-100, £29.50

The 100 LEDs produce the brightest light of all, this is with the light held 20 inches away.
The "20" is just about visible, but isn't very bright ((the wavelength isn't quite good enough)
but the entire note is lit, also showing the squiggly lines to the left (another UV security feature).

 

 

UV-1, £32.50

The sharp focused light shows up the "20" better than any other UV light.
This is with the light held 18 inches away.

Also UV-WHITE, £39.50, the UV light is the same as the UV-1, above, but it also has a very bright ordinary (white light) torch
and it has build-in rechargeable batteries that you recharge from any USB socket or with the mains adaptor supplied.

 

 

A CREDIT CARD

Here is an example of a good UV light and a 'bad' UV light when shone on a British twenty pound note.
Look for the two big letters, in this particular card it's "M" and "C" (not all credit cards have this feature).

 

UV-100, £32.50
I've used a strip of paper to cover the card number, you notice how it glows white.

 

UV9, £4.90
Please believe me, the "M" and "C" are not visible, not even if you look closely. If they were, they would show up the same brightness as the strip of paper.
You will also note that the strip of paper (the same card / strip of paper as pictured above) shows up a dull blue rather than bright fluorescent-white.

 

A RUBY

Finally, if you want a magnifier with a UV light, be extra careful which one you buy. Below are two sets of photographs showing a ruby, one with an unbranded 10X loupe with UV light (the size happens to be 10X21), one with the QUICKTEST 10X loupe with UV light. These loupes also have a rim of ordinary (white) light, you push the switch one way for white light, the other way for ultra violet light.

For a closer view, click on each of the two images.

395nm:
Firstly, the unbranded loupe (all four pictures are of the same loupe).
Top left: the loupe.
Top right: a ruby with the white light switched on, the ruby is dark, the light reflects off the surface.
Bottom left: note how pretty and bright the circle of ultra violet lights are.
Bottom right: the same ruby with the white light off and the ultra violet light switched on, it looks the same as the white-light picture.

10X21+UV

365-370nm:
Secondly, below, the QUICKTEST 10X20 loupe with UV light (all four pictures are of the same loupe).
Top left: the loupe.
Bottom left: note how there is only one ultraviolet light, not nearly as pretty as the rim of lights.
Top right: a ruby with the white light switched on, the ruby is dark, the light reflects off the surface.
Bottom right: the same ruby with the white light off and the ultra violet light switched on, it glows spectacularly. That's because there is a huge amount of ultraviolet light, even though the human eye cannot see it.

Please note that using ultraviolet light is not a 'test' for ruby, because some ruby (especially Burma ruby) fluoresces brilliantly, some ruby does not fluoresce at all; the above is just to show you the difference between the quality of ultraviolet lights (scroll up to read all about wavelengths).

 

 

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QUICKTEST, Watford, WD18 8PH, Tel. 01923 220206, email info(at)quicktest.co.uk